A semiconductor injection laser typically comprises a body of semiconductor material having an active layer between clad regions of opposite conductivity type. To increase the output power, a guide layer having a refractive index which is intermediate that of the active layer and a clad region may be interposed between one of the clad regions and the active layer. Light generated in the active layer propagates in both the active and guide layer thereby forming a larger beam at the emitting facet. Because of the refractive index difference between the clad regions and the active layer, or the combination of the active and guide layers, oscillation in the direction perpendicular to the plane of the layers is restricted to the fundamental transverse mode. In the lateral direction, a direction in the plane of the layers perpendicular to the direction of light propagation, a structural variation, such as one or more channels in the substrate, is used to form an optical waveguide which restricts the oscillation to the fundamental lateral mode. Lasers incorporating transverse and lateral waveguides such as those disclosed by Botez in U.S. Pat. No. 4,347,486 may have output powers in an excess of about 40 milliwatts (mW) in the fundamental lateral and transverse mode.
To increase the output power beyond that of an individual laser, monolithic arrays of spaced apart lasing regions, such as disclosed by Botez et al. in U.S. Pat. No. 4,547,396, have been fabricated with the lasing regions coupled to one another to form a phase-locked array operating as a single source. Arrays which operate in the single longitudinal mode at an output power as high as 80 mW have been observed. However, many of these devices exhibit a far field radiation pattern in a lateral direction consisting of two lobes symmetrically located about the normal to the emitting surface of the array. This far field radiation pattern is undesirable from a systems viewpoint because a large fraction of the beam cannot be used in systems with beam-forming optics.
As discussed by Butler et al., Applied Physics Letters 44, 293 (1984), this far field distribution can be understood from gain considerations that cause adjacent lasing regions to operate such that their relative phase is 180.degree.. It would thus be desirable to have a phase-locked laser array which included a means for reducing the likelihood of an array operating with 180.degree. phase difference between adjacent lasing regions.